A conventional technique for packaging an IC chip in a surface mount package is as follows.
A copper lead frame sheet is stamped from a thin copper sheet. The lead frame sheet contains an array of connected lead frames that will be eventually singulated.
The areas of the lead frame sheet that are to be wire bonded are plated with silver or other suitable material that does not readily oxidize, unlike copper.
IC chips are then bonded to the center pads of each lead frame, and a bonding machine wire bonds the pads of the IC chips to the top surfaces of the leads of each lead frame.
The lead frame sheet and IC chips are then encapsulated with a suitable material, such as resin, epoxy, plastic, or other material. The encapsulation may be performed by molding. The bottom surfaces of the leads that are to be later soldered to a printed circuit board (PCB) are masked during the encapsulation step.
The exposed portions of the encapsulated lead frame sheet are then plated in an electroplating process with a wettable metal, such as tin or a tin-lead compound.
The individual lead frames are then singulated by sawing through the sheet at the lead frame boundaries. This results in exposed copper sides of the leads.
During soldering of the leads to a PCB, the solder will not wet to the copper sides of the leads due to the copper being oxidized, but the solder will wet to the bottom surfaces of the leads (which are flush against pads on the PCB) since the bottom surfaces were plated with the wettable metal.
For packages where the leads do not extend out beyond the package body, such as quad flat no-lead (QFN) packages, dual flat no-lead (DFN) packages, and certain other types of surface mount packages, it cannot be determine from a top down view or a perspective view whether there has been a solder connection to a lead under the package, since the solder does not wet the sides of the leads. Therefore, a visual inspection of the solder connections cannot be quickly performed.
What is needed is a packaging technique that allows the sides of leads to be plated with a wettable metal so that, after the lead frame is soldered to a PCB, the solder will wet the sides of the leads. This will greatly simplify visual inspection of the solder bonds.